It has been known in the past to arrange an exhaust sensor in an exhaust passage of an internal combustion engine to detect a specific component in the exhaust gas (for example, see PLTs 1 to 3). The exhaust sensor described in PLT 1 is provided with an element body provided with an electrochemical cell, and a protective layer formed on the outer surface of the element body and comprised of a porous ceramic. The exhaust sensor is fastened to an exhaust pipe so as to be exposed to exhaust gas. Part of the exhaust gas passes through the protective layer and flows to the inside of the element body. Further, the exhaust sensor is provided with a heater for heating the element body so that the electrochemical cell becomes a predetermined operating temperature or more.
In this regard, when the temperature of the exhaust pipe is the dew point temperature of water or less, the water vapor in the exhaust gas condenses and condensed water is generated. If there is condensed water in the exhaust passage, drops of the condensed water strike the protective layer of the exhaust sensor together with the exhaust gas. If the protective layer does not have water repellency, the drops of water striking the protective layer will penetrate to the inside of the protective layer. If the temperature of the protective layer is high due to heating by the heater, the drops of water penetrating the protective layer will evaporate inside the protective layer. As a result, thermal shock will be given to the protective layer and element body and the element of the exhaust sensor will sometimes crack.
Therefore, PLT 1 describes preventing cracking of the element of the exhaust sensor by utilizing the Leidenfrost phenomenon to give the protective layer of the exhaust sensor water repellency. The “Leidenfrost phenomenon” is the phenomenon where when drops of water strike a high temperature protective layer, a film of water vapor is formed between the protective layer and drops of water whereby transfer of heat between the protective layer and the drops of water is suppressed. If the Leidenfrost phenomenon occurs, the drops of water are repelled from the protective layer, so water is kept from penetrating the protective layer.